Planar lightwave circuits (PLCs) are used to switch, attenuate, direct, modulate, separate, and combine light at different wavelengths. PLCs include waveguides and couplers formed in semiconductor wafers by photolithography. Advantages of large-scale manufacturability and compactness of PLCs are afforded by the methods borrowed from microelectronics and specifically developed for lightwave circuit applications.
One of most common examples of a PLC is an arrayed waveguide grating (AWG). AWGs are frequently used for wavelength separation and combination. An AWG includes an array of waveguides of incrementally increasing length between input and output slab couplers, to provide wavelength-dependent phase delay resulting in separation of light into individual wavelengths in the output slab coupler. Because the waveguides of the array have incremental lengths, an AWG typically has a bow-like, arcuate shape, with longer waveguides of the array disposed on the convex side of the AWG, and shorter waveguides of the array disposed on the concave side of the AWG.
To fit more bow-like waveguide structures on a single PLC wafer, it has been suggested to dispose individual AWG devices in a plane of the PLC wafer so that a convex side of one AWG borders a concave side of a next AWG. Referring to FIG. 1A, Watanabe et al. in US Patent Application Publication 2002/0113242 teach disposing individual AWGs 102 in two columns side by side on a PLC wafer 101, so that individual AWG chips 107 can be cut out of the PLC wafer 101 by making straight cut lines 103A and arcuate cut lines 103B. The technology for making arcuate wafer cut lines is readily available, e.g. sandblasting or laser machining can be used to cut the PLC wafer 101 along the straight cut lines 103A and arcuate cut lines 103B.
Since wafer processing costs are dominant in PLC production, disposing more AWG chips 107 on the single wafer 101 results in significant economical benefits. Thus, arcuate AWG chips 107 are less expensive to produce than rectangular AWG chips, not shown. The arcuate AWG chips 107 are also easier to heat and keep at a constant temperature, due to reduced thermal mass of arcuate-cut AWG chips as compared to rectangular-cut AWG chips. Despite these advantages, the usage of the arcuate AWG chips 107 has been hitherto hindered by an increased sensitivity of the arcuate AWG chips 107 to mechanical stress. In particular, mechanical stress directed along a line connecting arcuate ends of the arcuate AWG chip 107 (ends of the bow) creates a large optical path length variation in array waveguides of the AWGs 102, due to a photoelastic effect in the waveguides.
To reduce the impact of the mechanical stress, the arcuate AWG chips 107 can be strengthened. Referring to FIG. 1B, Watanabe et al. attached reinforcement beams 151 and 152 to reinforce the arcuate AWG chip 107, to make a reinforced AWG chip 107B. Alternatively, turning to FIG. 1C, Watanabe et al. disclose that an arcuate-cut additional silicon substrate 171 can be attached to the arcuate AWG chip 107, to make a compound arcuate AWG chip 107C. The reinforced arcuate AWG chips 107B and 107C shown in FIGS. 1B and 1C, respectively, are less sensitive to mechanical stress. Detrimentally, attaching additional materials and layers to the arcuate AWG chip 107 is time-consuming and costly. The reinforcing elements have to be matched in their thermal expansion to the AWG chip 107, to prevent bending of the AWG chip 107 out of its plane upon heating or cooling. Furthermore, the reinforced arcuate AWG chips 107B and 107C of FIGS. 1B and 1C, respectively, have a larger mass and thus have a larger thermal inertia, increasing a response time of a temperature stabilization loop.
The effect of optical path length variation due to squeezing or pushing the ends of arcuate AWG is so pronounced that attempts have been made to use this very effect for passive athermalization of AWG chips. For example, Ho et al. in US Patent Application Publication 2008/0080806 disclose an athermalized AWG having a stabilizing post or brace attached to ends of an arcuate AWG chip. The length of the post or brace changes with temperature, creating a temperature-dependent stress on the arcuate AWG chip, which counteracts thermal drift of the arcuate AWG chip. Similar ideas have also been disclosed in Chinese patent applications CN102354028A, CN101840030B, and CN100501468C.
One advantage of passive athermalization is that the temperature stabilization of the AWG chip is not required. Detrimentally however, passively athermalized AWG chips are more difficult to calibrate. Since every AWG has its own thermal drift magnitude, the length of the stabilizing post has to be individually adjusted for each AWG device manufactured, which is time-consuming, tedious, and impeding mass production.